In a digital imaging system, an amount of light received by an image sensor is adjusted, and an output characteristic with respect to the amount of received light is adjusted, to reduce a loss of shadow detail and a loss of highlight detail. An output image is processed, to keep a standard of quality of the image. When an X-ray imaging system is taken as an example of a digital imaging system, a dose control mechanism controls an X-ray source so that an input dose that reaches the image sensor is adjusted. An output value with respect to the input dose is adjusted by changing a gain of the image sensor. An X-ray image obtained depending on the adjusted output value is subjected to image processing such as compression/enlargement processing for a gradation number and contrast emphasis processing for emphasizing a particular parameter of the image.
As a technique for performing image processing according to a dose and an input/output characteristic, and suppressing an effect of a fluctuation in pixel value has been known. For example, Japanese Patent Application Laid-Open No. 2008-42448 discusses a technique for weakening, when a dose is frequently changed in moving image capturing, a response to image processing until the dose is stabilized. US 2006/0055991 discusses a method for changing a gain of a sensor according to a luminance range of a subject to fall within a sensitivity area of the sensor and performing gradation conversion using a function set for each gain. However, if an image that greatly deviates from a range in which an output characteristic is good is obtained, there is an issue in noise emphasis when a gradation is converted without considering the output characteristic.